In recent years, due to a global issue of carbon dioxide emission and worldwide increase of energy demand, saving automobile fuel has been demanded more and more. Under such circumstances, it has been demanded that a transmission (i.e., a component of an automobile) also contributes to fuel-saving more than ever.
For instance, one of fuel-saving methods of the transmission is lowering a viscosity of a lubricating oil. Among the transmission, a continuously variable transmission for an automobile is provided with a torque convertor, a wet clutch, a gear bearing mechanism, an oil pump, a hydraulic pressure controlling mechanism and the like. Lowering the viscosity of the lubricating oil used for such components reduces stirring resistance and friction resistance, thereby improving fuel efficiency of the automobile. However, the low-viscosity lubricating oil occasionally generates seizure due to a reduced shear stability and decreases the fatigue life.
In view of the above, Patent Literature 1 reports a lubricating oil composition capable of maintaining a gear shifting performance and the like for a long period of time, in which various additives are contained for optimization. However, since the invention disclosed in Patent Literature 1 is not directed to fuel-saving, a kinematic viscosity of the lubricating oil composition is high and a fatigue life thereof when the viscosity is lowered has not been studied.
Moreover, in order to further promote fuel-saving, decreasing the viscosity at low temperatures while maintaining the viscosity at high temperatures, in short, viscosity-index improvement has been demanded from the viewpoint of low-temperature startability.
For instance, a viscosity index of a lubricating oil composition disclosed in Patent Literature 2 is improved with a polymethacrylate (PMA) as a viscosity index improver. Moreover, in lubricating oil compositions disclosed in Patent Literatures 3 and 4, a viscosity property is improved with a high-viscosity synthetic oil (poly-alpha-olefin: PAO) and an olefin copolymer (OCP) effective for improving the fatigue life is further contained.